If I remember correctly. Every time there was a disruption on my award flights, I got miles.. Other than that, I took the original flights. So I thought people get miles when agent changes XY tickets.

Summer of 1998 (United)
I was on XY from Detroit to DEN. Agent changed my ticket and I got the miles.

1999 (United)
I was on XY PHX-DEN-EWR. Due to the weather in DEN, I was rerouted PHX-SFO-EWR. I got the miles.

July 2010 (American Airline)
I was on XY ATL-DFW-YYZ. I missed my connection at DFW. I was rerouted DFW-ORD-YYZ. I got AA miles.